"His ratings haven't been so high since he went to war with another former pro-Western neighbor: Georgia in 2008," Labott said.

Taking over territory

This is the scenario that Putin's fiercest critics worry is in the offing. They point to Russia's annexation of Crimea in March -- touted by Russia as a legitimate reflection of the will of the people and slammed by the West as a violation of Ukraine's sovereignty.

NATO warned this week that Russia could use "the pretext of a humanitarian or peacekeeping mission as an excuse to send troops into eastern Ukraine."

"We're not going to guess what's on Russia's mind, but we can see what Russia is doing on the ground -- and that is of great concern," NATO spokeswoman Oana Lungescu told Reuters in an emailed statement.

Clark, the former supreme allied commander of NATO, said even though it seems like Putin hasn't made up his mind yet about what steps he'll take, it's clear he has his eye on eastern Ukraine.

"He's building up his capacity to intervene. ... He wants that to be Russia," Clark said.

Protecting Russia

Others argue Putin could have another goal in mind if he sends troops into Ukraine: protecting his own country.

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It's true that Putin is weighing intervention, said Stephen Cohen, a professor emeritus of Russian studies at Princeton University and New York University.

But that doesn't mean it's a crisis of his making, said Cohen.

"He's being told by people who are advising him that this is no longer a struggle for Ukraine, but a struggle for Russia. These cities that are being attacked by the Ukrainian army are close to Russia," Cohen said. "He's being told, if you let these cities go, if you lose those cities, you will fight tomorrow in Russia."

"As Ukraine was slipping into anarchy and chaos, with all sorts of radicals causing mayhem, President Putin's endgame became obvious," he wrote. "He needed to do anything in his power to prevent Ukraine from becoming another Iraq, with a possibility of a civil war breaking out and violence spreading to Russia at some point."

Keeping Ukraine unstable

But could there also be an incentive for keeping things chaotic in the nation next door?

Plenty, Labott said.

"For years, Putin has made keeping Ukraine from joining the European Union and NATO a major strategic goal," she said.

But the February ouster of former Ukrainian President Viktor Yanukovych, who was closely allied with Russia, made that much tougher.

"One way to stop Ukraine from joining the West," Labott said, "is to make it too unstable by keeping this insurgency running."

Growing Russia's regional influence

Russia has spent billions of dollars in recent years building up its arsenal of warships, military planes and helicopters to go along with what remains the largest nuclear arsenal in the world, CNN's Jim Sciutto said.

Now come the reports of troops amassing near the Ukrainian border and renewed military exercises.

What's the point of flexing all this military muscle?

You don't have to look too far back in history to see an example of a similar situation that -- from Russia's point of view -- really paid off, Sciutto said.

"There's a view in Russia that that's when the world started taking Russia seriously again, because for years Russia had seen itself as being far behind, hopelessly behind the West in military terms. And that's a problem that Russia's leadership wanted to correct," said Sciutto, CNN's chief national security correspondent.

Sciutto said the focus is what Russia calls "the near abroad" -- former Soviet republics like Ukraine and Georgia.

"Russia wants to expand, reassert its influence (there) once again," Sciutto said. "And it feels it needs, in order to do that, to expand its military."